1. Field of the Invention
The present invention pertains to an improvement in buttress-threaded tubular connections. In particular, the connection is improved by controlling a particular combination of dimensional tolerances of the connection thread elements in a manner which provides a thread bearing contact pressure which is relatively constant over the entire length of the connection. In addition, the improved connection provides a reduction in hoop stress of about 20% over known buttress-threaded tubing connections machined to existing industry standard tolerances.
2. Background of the Invention
Threaded connections joining discreet lengths of steel pipe are used in many applications. The art of tubular connections is particularly well established for steel pipe used in the forms of tubing, casing, and drill pipe in the oil and gas well industry, and known collectively as oil country tubular goods (OCTG). There are numerous patents related to oil country tubular goods.
Oil and gas exploration and production companies have continued to expand the boundaries of depth, pressure, corrosive nature of fluid produced, and perhaps most important, economic criteria used to justify well development. These factors contribute to more stringent requirements for the material properties of the steel tubes used and the connections which join those tubes. For example, thirty years ago, users considered steel with yields of 80,000 psi to be high strength steel. Today, users routinely employ such steels and consider yields of 125,000 psi and higher not uncommon. Thus, it has become important to adjust connection attributes to function better in combination with the steels available today and under the production conditions and economic conditions of today.
Thirty years ago, at the time buttress threaded connections (joints) were improved to reduce the hoop stress developed toward the ends of connections or couplings, the machining of the connections was typically carried out using "chaser" type manual machine tools to achieve mass-production economies. Today, the development of computer-numerically-controlled (CNC) machines has led to the utilization of "single-point" cutting tools able to mass produce metal shapes. These CNC machines have the capability of far more accurately, and repeatedly, reproducing the intended design dimensions of any given connection. Coupled with advances in other technologies, such as precision measuring instruments, anti-galling metal treatments, and sophisticated assembly equipment, one can deliver a vastly improved product compared to the ones previously produced.
The goal of connection designers has been to develop connections capable of performing under demanding mechanical conditions which place the connection under high stresses, while providing a reliable seal against leakage of fluids through (across the threads of) the connection. In addition, it is highly desirable that the assembled connections be capable of being broken apart and reassembled without a significant reduction in performance characteristics of the connection.
U.S. Pat. No. 2,177,100 to William M. Frame, issued Oct. 24, 1939 discloses a leak-resistant pipe joint thread comprising a threaded female member, a complementary threaded male member engaging in threaded relation with the female member, and means providing a plurality of helically extending line seals between substantially each of the mating threads of the members. Preferably, the load flanks (trailing flanks) of the thread on the male and female members are surfaces bearing against each other, while clearances exist between at least portions of the remaining thread surfaces. The helically extending line seals are provided by a plurality of helically extending raised ribs on each root surface of the threads of the male and female members. Such raised ribs can be used on all thread surfaces other than the bearing load flank thread surfaces, if desired.
U.S. Pat. No. 2,772,102 to Samuel Webb, issued Nov. 27, 1956, describes a sealed, threaded pipe joint. The pipe joint comprises a "tapered non-upset, threaded connection characterized by 100% efficiency resulting solely from the screw-threaded contact". The joint comprises an internally threaded coupling member and an externally threaded pipe end member, the threads on each member having load-bearing and non-load-bearing flanks when tensile stresses are applied to the joint. In particular, the threads of the joint have a difference in phase, whereby a load-bearing flank of the pipe threads is in sealing engagement with a load-bearing flank of the coupling threads and a non-load-bearing flank of the pipe threads is in sealing engagement with a non-load-bearing flank of the coupling threads at points spaced apart a distance greater than the width of a fully formed thread but within the length of the fully formed threads. The combination of the pipe and coupling threads are substantially complementary and have fiat crests and roots, the threads being narrower than the thread grooves. The load-bearing flanks of the pipe and coupling threads are substantially normal to the longitudinal axis of the pipe and coupling. The roots and crests of the coupling threads are parallel to each other and on a taper relative to the longitudinal axis of the connection throughout the length thereof. The roots of the pipe threads are parallel to the roots and crests of the coupling and are on a taper relative to the longitudinal axis of the connection, throughout the length of the pipe threads; however, the crests of the pipe threads are parallel to the roots of the pipe threads for only a portion of the length of the pipe thread, providing a plurality of fully formed threads and vanishing threads, with the vanishing threads being at the junction with the unthreaded portion of the pipe. The crests of the coupling threads are parallel to the roots of the pipe threads and the roots of the coupling threads are parallel to the crests of the fully formed pipe threads, the mating threads on the coupling being at least as long as the total length of fully formed and vanishing threads on the pipe. The entire surface of the fiat crests of the coupling threads are in engagement with the roots of the pipe threads throughout the full length of the fully formed and vanishing pipe threads; the entire surface of the fiat crests of the pipe threads are in engagement with roots of the coupling threads throughout the full length of the fully formed threads on the pipe.
U.S. Pat. No. 3,109,672 to William F. Franz, issued Nov. 5, 1963 describes buttress threaded joints for oil well tubing which provide a drop in the hoop stress development toward the end of the joint coupling during power-make-up, permitting the safe use of higher working stresses. In particular, the threaded connection (pipe joint) disclosed comprises a pipe member having a cylindrical outer surface and a tapered buttress thread at the end thereof. The tapered buttress thread vanishes along the outer cylindrical surface of the pipe, providing a length of fully formed and a length of vanishing threads. The connection member corresponding with the threaded pipe member is a coupling member having a complementary thread on the internal surface thereof, the complementary thread being fully formed throughout the length thereof and at least as long as the total length of fully formed and vanishing threads on the pipe. The complementary threads on each connection member have following-flanks in bearing relationship and substantially normal to the longitudinal axis of the joint and leading-flanks in bearing relationship, wherein the leading-flanks have a larger flank-angle than the following flanks.
In particular, the following-flanks have a flank angle ranging between 0 degrees and 8 degrees for steel having a yield of 80,000 psi and greater and a flank angle ranging between 0 degrees and 1 degree for steel having a yield strength below 80,000 psi. The leading-flanks have a flank angle ranging between 30 degrees and 50 degrees, and preferentially an angle of about 45 degrees. This leading-flanks angle is necessary to prevent leakage of fluids (particularly gases) through (across the threads of) the connection, for reasons which will be discussed subsequently herein.
The thread crest and root truncations provide flat crests and roots which are parallel to the longitudinal axis of the joint, wherein the crest truncations of fully formed threads exceed the root truncations by an amount providing voids of predetermined amount between the crests and roots of the complementary threads throughout the length of the joint when the pipe and coupling are in hand-fight engagement. The crests of the coupling threads engage the roots of the vanishing pipe threads after power make-up of the joint to tightness, but the voids between the crests and roots of fully formed threads remain after the power make-up, and the initial voids between the coupling threads and vanishing pipe threads prevent the development of deleterious hoop stresses at the end of the coupling during power make-up.
The crest truncations of the fully formed threads exceed the root truncations by an amount providing voids of at least 0.002 inch. The voids cannot exceed 0.005 inches for oil well service wherein liquids only are involved and cannot exceed 0.0035 (preferably 0.003) when the fluid to be handled is predominantly gas. Since a void of 0.003 will achieve all of the objectives of the Franz invention, this was the amount of clearance void recommended. Use of a crest-root void of 0.003 inch between members results in at least about a 30% decrease in the diametrical interference developed at the end of the coupling.
Despite the desire to obtain clearance voids of 0.003 inch between fully formed complementary member threads, the machining techniques available at the time of the invention did not permit such accuracy in machined thread dimensions. For example, for a thread lead-flank angle of about 13 degrees, each 0.001 inch narrowing of the thread due to wear of the chaser tool decreased the crest root clearance by 0.0044 inch (a reduction exceeding the entire desired clearance void of 0.003 inch). This fact made use of a leading flank angel of 13 degrees impractical. Thus, the inventor recommended a leading flank angle of 45 degrees, since the loss in clearance void per each 0.001 inch of narrowing is only 0.0010 inch.
The specific threaded connection, described in U.S. Pat. No. 3,109,672 and commonly available throughout the world as a de-facto standard, uses a taper of 1 inch diametrical change for each 16 inches of longitudinal change (0.0625 inch per inch). The threads mate on the flanks rather than on root or crest, with the preferred embodiment of the invention exhibiting a thread form with roots and crests parallel to the longitudinal axis of the tube. The threads have a negative leading-flank angle (stab flank angle) of 45 degrees and a following-flank angle (load flank angle) of about zero degrees. These angles are defined in the geometric terms, which are subsequently used to describe the present invention. Under these geometric terms, a thread "flank angle" is understood to mean the angle formed between the thread flank and a line constructed at the root of the thread flank, which line is perpendicular to the longitudinal axis of the connection. For an externally threaded tubular member with increasing taper from right to left, a negative angle is one wherein rotation of the perpendicular line toward the thread flank is in a counterclockwise direction. A positive angle is one wherein the rotation is clockwise. These geometric terms do not comport with the terms used to describe flank angles in U.S. Pat. No. 3,109,672.
Problems have been encountered with the threaded connection of U.S. Pat. No. 3,109,672, because of unacceptably high levels of stress within the connection after make-up, due to interference during make-up between complementary member thread roots and crests. Frequently there is galling within the threads during make-up. In addition, secondary fluid leakage paths are created within the connection due to the galling within threads during make-up and the high levels of stress within the connection after make-up. When the amount of clearance between complementary member threads is increased to avoid the problems just discussed, more clearance than 0.006 diametric inch (0.003 inch per member thread) can result, and the pressure resistance capabilities of thread lubricant can be exceeded, rendering the connection unsuitable for service as a gas-tight connection. When the clearance exceeds 0.010 diametric inch, the connection will not even contain liquids.